do-while

(PHP 4, PHP 5)

do-while loops are very similar to
while loops, except the truth expression is
checked at the end of each iteration instead of in the beginning.
The main difference from regular while loops is
that the first iteration of a do-while loop is
guaranteed to run (the truth expression is only checked at the end
of the iteration), whereas it may not necessarily run with a
regular while loop (the truth expression is
checked at the beginning of each iteration, if it evaluates to
FALSE right from the beginning, the loop
execution would end immediately).

There is just one syntax for do-while loops:

<?php$i = 0;do { echo $i;} while ($i > 0);?>

The above loop would run one time exactly, since after the first
iteration, when truth expression is checked, it evaluates to
FALSE ($i is not bigger than 0) and the loop
execution ends.

Advanced C users may be familiar with a different usage of the
do-while loop, to allow stopping execution in
the middle of code blocks, by encapsulating them with
do-while (0), and using the break
statement. The following code fragment demonstrates this:

Don't worry if you don't understand this right away or at all.
You can code scripts and even powerful scripts without using this
'feature'.
Since PHP 5.3.0, it is possible to use
goto
operator instead of this hack.

User Contributed Notes 8 notes

There is one major difference you should be aware of when using the do--while loop vs. using a simple while loop: And that is when the check condition is made.

In a do--while loop, the test condition evaluation is at the end of the loop. This means that the code inside of the loop will iterate once through before the condition is ever evaluated. This is ideal for tasks that need to execute once before a test is made to continue, such as test that is dependant upon the results of the loop.

Conversely, a plain while loop evaluates the test condition at the begining of the loop before any execution in the loop block is ever made. If for some reason your test condition evaluates to false at the very start of the loop, none of the code inside your loop will be executed.

I'm guilty of writing constructs without curly braces sometimes... writing the do--while seemed a bit odd without the curly braces ({ and }), but just so everyone is aware of how this is written with a do--while...

Also, a practical example of when to use a do--while when a simple while just won't do (lol)... copying multiple 2nd level nodes from one document to another using the DOM XML extension

<?php# open up/create the documents and grab the root element$fileDoc = domxml_open_file('example.xml'); // existing xml we want to copy$fileRoot = $fileDoc->document_element();$newDoc = domxml_new_doc('1.0'); // new document we want to copy to$newRoot = $newDoc->create_element('rootnode');$newRoot = $newDoc->append_child($newRoot); // this is the node we want to copy to

# loop through nodes and clone (using deep)$child = $fileRoot->first_child(); // first_child must be called once and can only be called oncedo $newRoot->append_child($child->clone_node(true)); // do first, so that the result from first_child is appendedwhile ( $child = $child->next_sibling() ); // we have to use next_sibling for everything after first_child?>

I've found that the most useful thing to use do-while loops for is multiple checks of file existence. The guaranteed iteration means that it will check through at least once, which I had trouble with using a simple "while" loop because it never incremented at the end.

My code was:

<?php$filename = explode(".", $_FILES['file']['name']); // File being uploaded$i=0; // Number of times processed (number to add at the end of the filename)do {/* Since most files being uploaded don't end with a number, we have to make sure that there is a number at the end of the filename before we start simply incrementing. I admit there is probably an easier way to do this, but this was a quick slap-together job for a friend, and I find it works just fine. So, the first part "if($i > 0) ..." says that if the loop has already been run at least once, then there is now a number at the end of the filename and we can simply increment that. Otherwise, we have to place a number at the end of the filename, which is where $i comes in even handier */

/* Now that everything is uploaded, we should move it somewhere it can be accessed. Hence, the "uploaded" folder. */move_uploaded_file($_FILES['file']['tmp_name'], "uploaded/".$filename[0].".".$filename[1]);?>

I'm sure there are plenty of ways of doing this without using the do-while loop, but I managed to toss this one together in no-time flat, and I'm not a great PHP programmer. =) It's simple and effective, and I personally think it works better than any "for" or "while" loop that I've seen that does the same thing.

If you are trying to use a construct like this:<?phpdo {// some code to run only one more time if expression is trueif ($your_expression == true) continue;} while (false);?>It will NOT loop as expected, because the continue tries to run the "next" loop, but the expression says just "false" so there is no next loop. The continue exits the while loop.To get around this you may use an other expression, like this<?phpdo {// some code to run only one more time if expression is trueif ($your_expression == true) continue; break;} while (true);?>or use the goto statement since PHP 5.3.